1. Field of the Invention
The present invention relates to novel polyurethane polymers useful in the production of materials having nonlinear optical activity.
Materials displaying nonlinear optical activity may have a strong cubic (.chi..sup.3) or quadratic (.chi..sup.2) susceptibility. This invention more especially relates to polymers or materials having a high quadratic susceptibility after polarization, and which also have a good cubic susceptibility.
2. Description of the Prior Art
Numerous polymers are known to this art that possess the property of being nonlinearly optically active. In certain cases, this activity is greater than that of the inorganic materials currently used for such purpose.
Polymers used in NLO (nonlinear optics) generally contain conjugate groups with dislocated electrons, which provide them with a significant cubic .chi..sup.(3) susceptibility value.
Furthermore, in certain polymers (including those of the present invention), the groups responsible for the NLO activity are not centrosymmetrical and have a strong hyperpolarizability .beta. of the second order.
These noncentrosymmetrical groups are most typically conjugated groups with transfer of charges, the orientation of which within the materials by a polarizing electrical field renders the material noncentrosymmetrical. The material then has a nonzero quadratic susceptibility .chi..sup.(2).
Heretofore, these polymers were polymers with conventional backbones, such as the polyolefins, polyacrylates, polymethacrylates, polychloroacrylates or polysiloxanes, for example, onto which polar charge transfer groups were grafted.
Compare, for example, EP-No. 262,680 and FR-No. 597,109. However, these polymers have certain disadvantages and, in particular, difficulties are encountered in the preservation of their nonlinear optical activity. Indeed, the graft segments must have a certain mobility to enable their orientation in an electrical field. But they always retain a certain residual mobility, whereby over the course of time they may lose their orientation. This results in a decrease in their electronic centrosymmetry generated by the electrical field and, consequently, in their nonlinear optical activity. This phenomenon is illustrated in the article by C. Ye et al, "M.R.S. Symposium Proc.", Vol. 109 (Nonlinear Opt. Proc. of polymers, p 263) J. Heeger Editor, 1988.
Another disadvantage of these polymers resides in the fact that the number of polar groups with charge transfer is generally low, as it is a function of the structure of the polymer. Furthermore, the number of grafts cannot be very high without introducing a significant change in the properatives of the polymer.
Materials are also known to the art which possess nonlinear optical activities and typically are constituted by a polymer matrix, into which is dissolved a compound having a nonlinear optical activity cf the conjugate/charge transfer molecule type.
However, these compounds generally have low solubilities in the matrix and also possess a residual mobility in such matrix, which in time will effect a disorientation of said compounds, which renders the material centrosymmetric relative to electrical charges.
In fact, the quadratic nonlinear optical activity is generated by the dislocation of the electrons and a substantial noncentrosymmetry of the charges at the molecular and material level.